1. Field of Invention
The present invention relates to data networks and, in particular, to a burst-switching network with rate-regulated transfer of data.
2. Description of the Related Prior Art
Since its inception in the nineteenth century, the circuit-switched telephone network provided a high-quality service where a path of fixed capacity, from a traffic source to a traffic sink, is guaranteed during a connection period. Circuit switching, however, was considered unsuitable for data communications. Unlike voice communications, data transfer tends to be sporadic, thus leading to poor utilization of a circuit-switched connection of fixed capacity. This led to the concept of packet switching where data are organized in packets of arbitrary lengths, each packet carrying in its header sufficient information to enable its routing through a packet network. With uncoordinated packet sources and unknown data rates, successful transfer of packets in a packet network cannot be guaranteed and several techniques, well known in the art, were developed to reduce the probability of packet loss en route.
In a network where a data stream traverses intermediate nodes, rate regulation need be applied only at the source node. However, each intermediate node must still forward the individual packets of the data stream. To reduce the packet-forwarding effort, it is beneficial to aggregate the packets of a data stream into data bursts, each data burst comprising a relatively large number of packets; 160 for example. A major justification for packet aggregation is the currently available high-capacity optical channels. A packet of 150 bytes transferred over a channel of 150 Mb/s capacity has a duration of 8 microseconds. A packet of 10,000 bytes has the same duration of 8 microseconds on a 10 Gb/s channel. While aggregation is desirable in a network employing electronic core nodes, it is necessary in a network employing optical core nodes. The switching latency of a fast optical switch is likely to be of the order of 100 nanoseconds while a packet of 150 bytes has a duration of only 120 nanoseconds in a 10 Gb/s channel. Thus, if individual packets are switched in an optical core node, a significant proportion of channel capacity and switch capacity would be wasted. In addition, because optical switches are currently bufferless, the transmission of data packets at the edge nodes must be precisely timed to arrive at an optical switch at pre-calculated instants of time and the use of aggregated packets, i.e., data bursts, would significantly reduce the time-coordination effort.
Providing reliable services in a data network requires end-to-end paths of controllable capacity allocation (flow-rate allocation). Much of the work done in this area focused on the transfer of data blocks of fixed size, as in Asynchronous-transfer mode (ATM) communications where several devices were developed to regulate the transfer of ATM cells. There is a need, however, for a device to realize flow-rate regulation in a network transferring variable size packets or data bursts where each burst may comprise several packets. Such a device must be scalable to handle a very large number of data streams of diverse flow-rate requirements and be adapted for use in an edge node or in a core node. The flow-rate allocations can be dynamic and the envisaged device must, therefore, be adapted to handle time-varying flow-rate allocations.
In U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 by the present inventors and titled “Rate Regulated Burst Switching”, a method and apparatus are provided for low latency loss-free burst switching. Burst-transfer schedules are initiated by controllers of bufferless core nodes and distributed to respective edge nodes. In a composite-star network having edge nodes interconnected by independent core nodes, the burst-transfer schedules are initiated by any of a plurality of bufferless core nodes and distributed to respective edge nodes. Burst formation takes place at source nodes and a burst size is determined according to an allocated flow-rate of a burst stream to which the burst belongs. An allocated flow-rate of a burst stream may be modified according to observed usage of scheduled bursts of a burst stream. A method of control-burst exchange between each of a plurality of edge nodes and each of a plurality of bufferless core nodes enables burst scheduling, time coordination, and loss-free burst switching. The method of the above patent application requires that a controller of each optical core node have a burst-description generator driven by a flow-rate regulator.
A network providing optical burst switching in the core requires flow-rate regulation at the electronic edge nodes to enable contention-free switching at subsequent core nodes. The bursts are generally of arbitrary sizes and switching at the electronic edge nodes requires burst segmentation into data segments of equal size, with a proportion of the data segments including null data. Prior-art flow-rate regulation methods do not take into account the data composition within switched data segments, thus compromising the accuracy of flow-rate control.
There is a need, therefore, for methods and apparatus for regulating the flow of a large number of streams of variable-size data packets or bursts based on flow-rate allocations that are adapted to time-varying traffic conditions. The apparatus need also be coordinated with scheduling devices in both edge nodes and core nodes. Where data packets or bursts are segmented to facilitate switching, the flow control must be based on the actual information content in the switched data segments. Such an apparatus would enable reliable burst switching with service-quality control.